Ion implantation is an important process in semiconductor/microelectronic manufacturing. The ion implantation process is used in integrated circuit fabrication to introduce dopant impurities into semiconductor wafers. An ion-source is used to generate a well-defined ion beam for a variety of ion species from a dopant gas. Ionization of the dopant gas generates the ion species which can be subsequently implanted into a given workpiece.
Isotopically enriched dopant gases have emerged as widely used dopant gas precursors in the semiconductor industry. As used herein and throughout the specification, the terms “isotopically enriched” and “enriched” dopant gas are used interchangeably to mean the dopant gas contains a distribution of mass isotopes different from the naturally occurring isotopic distribution, whereby one of the mass isotopes has an enrichment level higher than present in the naturally occurring level. By way of example, 58% 72GeF4 refers to an isotopically enriched or enriched dopant gas containing mass isotope 72Ge at 58% enrichment, whereas naturally occurring GeF4 contains mass isotope 72Ge at 27% natural abundance levels.
By using a dopant gas at a given flow rate that is enriched in a desired atomic species to be implanted, an ion beam is produced with higher beam current in comparison with a corresponding non-enriched dopant gas utilized at the same flow rate. As an example, for a given flow rate, 58% 72GeF4 enriched in mass isotope 72Ge at 58% enrichment can produce a beam current twice as high as a beam current produced with naturally occurring GeF4 having mass isotope 72Ge at 27% natural abundance levels. In other words, twice as many 72Ge ions are generated in the ion chamber per unit volume of flow rate when utilizing 58% 72GeF4 as compared to naturally occurring GeF4. In addition to higher beam current, the enriched dopant gas allows the ability to achieve the required implant dosage of the desired atomic species utilizing less dopant gas compared to the corresponding non-enriched dopant gas.
Such process benefits are particularly advantageous when the enriched dopant gas is a fluorine-containing gas, which is well known to etch walls of a tungsten ion chamber and form tungsten fluoride (WFx) species that can migrate to the hot source filament where tungsten can be deposited. The tungsten deposits can cause momentary drops in beam current known as glitching or arc discharge. Ultimately the ion source performance may deteriorate and degrade over time when the glitching and/or frequency of glitching reaches an upper threshold such that the ion apparatus cannot operate with acceptable efficiency. In such a scenario, the user must stop the implant operation and perform maintenance to clean the deposits or replace the ion source. Such downtime results in productivity loss of the ion implantation system. Consequently, the ability to achieve the required implant dosage of the desired atomic species by using a correspondingly enriched fluorine-containing dopant gas can introduce less fluorine into the ion source chamber, thereby reducing tungsten deposits and potentially extending ion source life.
Furthermore, use of various enriched dopant gases at the same flow rate as its non-enriched dopant gas offers the advantage of increased throughput and yield. More dopant atomic species are available for implantation per unit flow of enriched dopant gas, as compared to its non-enriched dopant analog. As a result, the potential for productivity and yield increases when enriched dopant gases are utilized.
In addition, isotopic enrichment of a desired atomic species to a sufficient level for producing the required ion implant dosage can eliminate cross-contamination problems with other species. Today, many of the ion source tools are non-dedicated, meaning that several different atomic species are implanted using the same tool. By way of example, naturally occurring or non-enriched germanium tetrafluoride (GeF4) has been used for implanting 74Ge ionic species because it is the most abundant at 37% in naturally occurring GeF4 (i.e., GeF4) compared to the other stable isotopes of GeF4 (i.e., 70Ge, 72Ge, 73Ge and 76Ge). Many of these source tools also process and implant 75As, which creates cross-contamination problems with 74Ge, as the ion magnetic separator does not recognize the difference between the two atomic species by virtue of only a 1 atomic mass unit (a.m.u.) difference between the 75As and 74Ge species. In other words, the ion source chamber cannot resolve or filter the residual 75As in the resultant ion beam from previous production runs, as the mass resolution of conventional beams are not capable of recognizing the difference in 74Ge and 75As atomic species. As a result, both 74Ge and 75Ge species can be inadvertently implanted into a workpiece, thereby rendering the microelectronic device contaminated and potentially not fit for its intended purpose. It has been shown that contamination can reach 6% or higher of the intended germanium dose.
To alleviate such cross-contamination, it has been recognized that isotopically enriched Ge atomic species that are 2 or more a.m.u. away from 75As can be utilized. As such, isotopic enrichment of naturally occurring GeF4 in 72Ge can raise the concentration of 72GeF4 whereby the mass 72 germanium isotope is enriched from its natural abundance level of 27% to about 52% or higher such that the required Ge dosage can occur without 75 As cross-contamination. The enriched levels of 72GeF4 achieve the required dosage in a manner that introduces less overall gas. Additionally, less consumption of the enriched 72GeF4 also translates into less fluorine ions available for etching chamber components and depositing tungsten onto source filaments.
Such process benefits have prompted users such as semiconductor fabs and foundaries to replace conventional non-enriched dopant gases in their ion implant processes with corresponding enriched and highly enriched dopant gas analogs. Generally speaking, in the microelectronics industry, because the ion source tools have previously been qualified to operate at established processing parameters demonstrating the ability to precisely and reliably produce acceptable wafers with the required implant dosage, the ion implantation processing parameters preferably are to remain unchanged when utilizing the corresponding enriched dopant gases.
However, notwithstanding the above mentioned process benefits, semiconductor fabs and foundaries are encountering difficulties using the enriched dopant gas analogs in a manner that does not significantly depart from the previously qualified operating parameters of the ion source tool. In particular, operating the ion source tool at the same flow rate of enriched dopant gas as previously used with the non-enriched or lesser enriched dopant gas increases the beam current to a level that represents a departure from the qualified ion beam current. As a result, the entire ion implantation process must be re-qualified, which is a time consuming process effectively amounting to unacceptable production downtime. Additionally, the increased ion beam current is particularly problematic for enriched fluorine-containing dopant gases in which the ionization of the enriched dopant gas produces a large amount of free available fluoride ions that can form various ionic species of tungsten fluoride (WFx). As mentioned, WFx species tend to migrate to hotter surfaces in the ion source chamber, including the cathode or source filament, where they can deposit tungsten and potentially cause glitching and short the ion source. As tungsten deposits, the atomic or molecular fluorine is released and is available to continue the so-called “halogen cycle” of etching additional tungsten walls. As a result, end-users are observing that employing a fluorine-containing enriched gas may in some instances shorten source life by accelerating the halogen cycle.
The aforementioned problems are potentially compounded when utilizing higher levels of enriched dopant gases. Accordingly, ion implant users may not be achieving the process benefits of isotopically enriched dopant gases originally intended. There is an unmet need for an ion implantation process that can realize the process benefits of utilizing enriched dopant gases that eliminates the aforementioned problems.